1. Technical Field
This disclosure relates to a semiconductor sensor using a piezoresistor such as a semiconductor acceleration sensor and a semiconductor angular velocity sensor.
2. Description of the Related Art
Semiconductor sensors are used for measuring, for example, acceleration applied to moving vehicles in the moving direction or the lateral direction, camera shake, and so forth.
Some semiconductor sensors include piezoresistors formed on the surfaces of single crystal silicon wafers using the same techniques and methods for fabricating ICs (integrated circuits) so as to use the piezoresistors as strain gauges (see, for example, Patent Documents 1, 2, and 3). In this type of semiconductor sensor using a piezoresistor, a piezoresistor is formed on one side of a silicon wafer, while a gap is formed in the other side of the silicon wafer by, e.g., etching so as to form a thin beam under the piezoresistor. With this configuration, as the resistance of the piezoresistor changes in response to bending of the beam, the semiconductor sensor can obtain electric signals corresponding to acceleration by measuring the resistance of the piezoresistor.
The semiconductor sensor is provided with a proof-mass so as to make the beam more easily flexed. The beam is fixed at one end to the proof-mass and at the other end to a frame surrounding the proof-mass. A metal wiring pattern and a pad electrode electrically connected to the piezoresistor are formed on the frame.
It has been proposed that a cover plate for limiting displacement of the proof-mass be disposed spaced apart from the proof-mass and the beam in order to prevent the beam from being damaged by a strong impact (see, for example, Patent Document 4).
In a semiconductor sensor disclosed in Patent Document 4, gaps for fixing a cover plate are provided in the surface of a frame. The cover plate is fixed by the gaps such that the cover plate is located above a proof-mass and a beam and limits displacement of the cover plate.
However, having such dedicated areas for fixing the cover plate in the frame results in increasing the chip area of the semiconductor sensor. To avoid an increase of the chip area of the semiconductor sensor, the cover plate may be bonded to the frame using epoxy adhesive. However, epoxy adhesive contracts with the heat of bonding and generates a residual stress, which adversely affects the sensitivity of the semiconductor sensor. Moreover, there are other problems with using epoxy adhesive, such as non-uniform application of epoxy adhesive and application of epoxy adhesive to unintended areas.
In an alternative method, glass frit bonding is used in place of adhesive. However, even if glass frit bonding is used, the problem of generation of residual stress due to heat contraction during bonding occurs.
This problem may be solved by bonding the cover plate to the frame by using anodic bonding.
FIG. 7A is a plan view showing an example of a related-art semiconductor sensor 1a. FIG. 7B is a cross-sectional view taken along line A-A of FIG. 7A. FIG. 7C is a cross-sectional view showing the semiconductor sensor during anodic bonding.
The semiconductor sensor 1a comprises a SOI (Silicon-on-Insulator) substrate 2 including a silicon layer 2a, an insulation layer 5 formed under the silicon layer 2a, and a silicon layer 2b formed under the insulation layer 5.
A frame-like frame 6 is formed of the SOI substrate 2. Beams 8 formed of the silicon layer 2a are provided to extend continuously from the upper surface side of the SOI substrate 2. Piezoresistors 10 are provided in the silicon layer 2a of the beams 8.
A proof-mass 4 is disposed spaced apart from the frame 6 in the center of the area surrounded by the frame 6. The proof-mass 4 is connected at the upper surface side to the beams 8 via the silicon layer 2a and is supported by the beams 8.
An insulation film 12 is formed on the upper surface of the SOI substrate 2. In FIG. 7A, the piezoresistor 10 are shown for explanation purposes. Plural metal wiring patterns 20 and plural pad electrodes 16 are formed on the insulation film 12, and are electrically connected to the piezoresistors 10 via the through holes 12a formed in the insulation film 12.
Although not shown in FIG. 7A, a protection film 21 is formed on the insulation film 12, covering the metal wiring patterns 20. Openings are formed in the protection film 21 on the pad electrodes 16 so as to expose the surfaces of the pad electrodes 16.
A glass substrate 3 is bonded to the lower surface of the frame 6 by anodic bonding. The lower surface of the proof-mass 4 is spaced apart from the glass substrate 3.
A cover plate fixing area 18 is provided at the periphery of the surface of the frame 6. A cover plate 24 made of a glass substrate is fixed to the cover plate fixing area 18 by anodic bonding. A gap 24a is formed in the center of the lower face thereof such that the bottom surface of the gap 24a is spaced apart from the proof-mass 4 and the beam 8.
<Patent Document 1> Japanese Patent No. 2670048
<Patent Document 2> Japanese Patent Laid-Open Publication No. 2004-233080
<Patent Document 3> Japanese Patent Laid-Open Publication No. 2004-257832
<Patent Document 4> Japanese Patent Laid-Open Publication No. 2004-233072
In the related-art semiconductor sensor 1a shown in FIGS. 7A-7C, the cover plate 24 is bonded, by anodic bonding, to the cover plate fixing area 18 provided on the frame 6. Anodic bonding is simpler than bonding using adhesive or glass frit bonding and provides stable bonding strength.
When fixing the cover plate 24 to the frame 6 by anodic bonding, electrostatic force is generated by a DC current applied during the bonding. Thus, the proof-mass 4 is attracted to the cover plate 24 by the generated electrostatic force and sticks to the bottom surface of the gap 24a (see FIG. 7C). Even if the bottom surface of the gap 24a of the cover plate 24 is roughened, sticking of the proof-mass 4 cannot be completely prevented.
Once the proof-mass 4 sticks to the cover plate 24 due to anodic bonding, the proof-mass 4 cannot be easily separated from the cover plate 24 even by applying an electric field of opposite bias to the bias for bonding or by removing the electrostatic force. Although the proof-mass 4 can be separated from the cover plate 24 with vibration energy using ultrasonic cleaning or fine-jet cleaning, these methods have a high risk of breaking the beams 8 and may reduce the production yield of the semiconductor sensor 1a. 
The sticking of the proof-mass 4 to the cover plate 24 may be prevented by increasing the distance between the proof-mass 4 and the cover plate 24. However, in the case where the proof-mass 4 is attracted to the cover plate 24 by electrostatic force during bonding, the beams 8 might be broken.
The contact area between the proof-mass 4 and the cover plate 24 can be reduced by providing the gap 24a of the cover plate 24 with local depth variation. However, as a process for providing the local depth variation is required, the production cost of the semiconductor sensor 1a is increased.